Open Ponds Versus Closed Bioreactors

There are pros and cons associated with the economic commercial production of algae using closed bioreactors and open ponds. Is one method superior, or is there room for both?

By Anna Austin

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The U.S. DOE's National Renewable Energy Laboratory concluded in 1990 in its Aquatic Species Program close-out report that open raceway ponds were the most viable solution for the mass production of algae for conversion into biofuels, but that it was much too early to determine whether open, closed or hybrid designs of growing algae would ultimately prevail.

Generally, open ponds have been associated with contamination issues, excessive space requirements and limited location possibilities due to climate. At the same time, closed bioreactors have mainly been considered too expensive. There wasn't much room for doubting the accuracy of NREL's report, but have technological advancements in the past two decades leveled the playing field? Perhaps, but companies today pursuing either route still face the same hurdles their predecessors did. Whether it takes five, 10 or 20 years, the key to economic algae-based biofuel production is developing the most cost-effective growth model possible.

If light limitation is the main problem in achieving the commercial potential of algae in scaled commercial cultivation operations, Massachusetts-based Bodega Algae may have the solution, according to CEO Joseph Dahmen. In January, Bodega Algae and Bigelow Laboratory for Ocean Sciences in West Boothbay Harbor, Maine, received a six-month, $150,000 Small Business Innovation Research grant from the National Science Foundation to develop and test a prototype for growing high concentrations of algae for use as biofuel. More specifically, Bodega will use the funds to develop advanced photobioreactors, and is making "big advancements," Dahmen says. Bodega Algae will work on the grant with Bigelow Laboratory for Ocean Sciences in West Boothbay Harbor, Maine.

Case Closed

One of the major issues in the cultivation of microalgae is light limitation, Dahmen says. "This limits the effective photosynthetic zone to the volume within five centimeters of the surface of a pond," he says. "Everything below that tends to be light prohibited because the top layer limits the light from getting in." The same is true for photobioreactors, he adds. "So some people have tried various solutions like flat plates or hanging bags, and in effect, what they've done is limit the cultivation volumes in an attempt to drive up the surface area to volume ratio."

These small volumes allow light to penetrate better, according to Dahmen, but the problem is that it may lead to biofouling (the attachment of organisms to a surface in contact with water for a period of time) and the cost of pumping the algae around through the small volumes increases. "We're bringing the light to the algae with some proprietary optics located within the reactor," Dahmen says. "Cultivation volumes that are lit internally allow us to cultivate very efficiently in three dimensions."

The bioreactors Bodega is currently experimenting with are bench units made of acrylic. In the long term, however, the company is looking at shipping containers and possibly petroleum distilate storage tanks.

Dahmen describes open ponds as a "first-generation solution" to growing algae. "They're very land intensive because the effective cultivation area is limited to a very thin slice of growth medium, so the ponds have to expand, becoming very land hungry," he says. "Also, if you look at the areas receiving high amounts of natural sunlight or insolation where ponds make the most sense, you run into tremendous problems with evaporation as well as cross-contamination of cultures. When you start talking about acres and acres of ponds 16 inches deep, you've increased the surface area to the point where land consumption is a huge problem."

Open ponds are relatively cheap to build compared with bioreactors, though, Dahmen says. "But what we're seeing is a real need for cost-effective photobioreactors that can address the capital expense issues while offering efficient cultivation in large volumes."

Numerous other companies share Dahmen's perspectives, but have approached bioreactors in different ways. Solix Biofuels, recently named a part of the U.S. DOE's $44 million National Alliance for Advanced Biofuels and Bioproducts consortium, has attracted much attention in the past few years. Along with Colorado State University, Solix has developed specialized photobioreactor systems composed of long, closed plastic bags containing algae, which float in large water-filled metal tanks to control temperature and are injected with CO2 through tubing to optimize growth.

California-based OriginOil, another bioreactor contender, has a cooperative agreement with the U.S. DOE's Idaho National Laboratory for a multiphase algae research program. The company describes its Helix BioReactor as an advanced algae growth system that features a rotating vertical shaft with low-energy lights arranged in a helix/spiral pattern, resulting in a theoretically unlimited number of growth layers.

While these particular companies have focused on bioreactor development, some such as Washington-based Bioalgene Inc. have pursued both methods.

Open to Possibilities

A few years ago, aircraft manufacturer Boeing hired Bioalgene to survey indigenous strains of algae-regional strains that grow fast and produce many lipids-in the Northwest U.S., according to Bioalgene CEO Stan Barnes. The company has leased a decommissioned wastewater plant where it is now testing selected strains. "These are natural strains that already have defense mechanisms against predators and disease and can thrive in this region," Barnes says. Now entering phase two of its research project, Bioalgene will grow algae in larger, 220,000-gallon ponds on a five-acre tract at Boardman, Ore., to test variances in growing and harvesting methods.

Barnes says early on, the company built three bioreactors at Seattle University, and though being able to grow pure strains was an advantage, capital costs to build, maintain and clean transparent systems didn't seem to be an economic pathway to high-volume algae production. Using NREL's research as a basis for the company's decision to move forward with natural strains in open ponds, Barnes says Bioalgene utilized the already developed capabilities of algae to yield a simple system, rather than a complex system. "Evaporation is one of the things we're concerned about though," he tells Biomass Magazine. "The whole question of water management is a challenge, and I think you'll have it anywhere. One big advantage a closed system has is no evaporation loss."

Adequate temperature and sunlight are only available in certain regions for limited periods of time, but Barnes says one of the benefits Bioalgene will reap by growing algae at a coal-fired power plant (besides using flu gas emissions to accelerate growth) is that the process heat allows growth into December by warming water that is fed to the algae. "As long as the water is warm, there is plenty of light energy to keep the algae growing," he says.

Although Bioalgene believes open ponds are the ultimate solution, it will utilize closed reactors as nurseries to grow inoculation strains in pure forms before introducing them to ponds. "Overall, the potential for volume, we see, is more economical (in open ponds) than in large closed systems," he says. Bioalgene expects its systems to be able to deliver more than 100,000 tons of algae per year.

But what if the algae are being produced for something other than oil? Jim Oyler, CEO of Utah-based Genifuel Corp., says the method of growing algae is relative to the intended use. Algae oil developers are looking to achieve the highest yields of oil possible using specific strains, but oil yields aren't important to Genifuel, as it is directly converting the algal biomass to natural gas via a gasification process developed by the DOE's Pacific Northwest National Laboratory.

Room for Both

Though Genifuel is focused on mass rather than oil yields, growing the material as cheap and quickly as possible is imperative. The company has open raceway ponds in Utah, which are currently shut down for the winter months, but produced algae last year. "In our case, we're interested in growing the most biomass possible per unit of area in our ponds, so our goal is different than the goal of algae oil producers," Oyler says. "We like fast-growing species and in many cases these are tough, aggressive types of algae. Many of the oil producers, especially when they are genetically modified, can be somewhat delicate or vulnerable, and are easily taken over by weeds."

Most oil producers will make the case that they can get faster growth in bioreactors, while at the same time avoid problems that arise from outdoor production, including susceptibility to parasites and the potential for aggressive species to take over. "The key question is, can you get enough additional productivity in bioreactors to offset the additional cost?" Oyler says. "There are some very clever designs being developed. Solix Biofuels has a design that's not too expensive-more expensive than open ponds-but it reduces capital costs in a productive way."

Open ponds have not always been consistent, however, as they have peak productivities that aren't maintainable or achievable in most climates over extended periods of time. "There are also some very clever designs that overcome some of those problems," Oyler says. "But in order for bioreactors to pay off, they're going to have to achieve something in the order of double or triple the productivity of an outdoor open pond. It's yet to be proven that it can be done. Theoretically it might be possible, but no one's actually demonstrated it at a commercial scale."

Another advantage of closed systems is that they open up sunny, dry areas such as the Southwest to biofuel production. Open ponds are unlikely to work in the Southwest because the water loss is going to be enormous, Oyler says. "Photobioreactors keep water enclosed, but thermal management is still needed, because if you put an enclosed system out in the desert it's going to get really, really hot in there."

Although the problems of predators and weeds have been solved with bioreactors, such closed systems used to grow algae for other purposes have experienced problems with virus susceptibility and/or bacteria attacks, which can take the whole system down in a matter of hours. "There are ways to deal with that, but I don't believe that it has ever been fully solved for long periods of time," Oyler says. "Both ponds and bioreactors have advantages and disadvantages right now. There's more experience with outdoor systems, but the closed systems have the promise of higher sustained productivity, but only if they can overcome associated problems, especially thermal management or diseases."

Al Darzins, principal group manager of NREL's National Bioenergy Center, shares Oyler's sentiment. During the past couple of years, algae research has enjoyed a resurgence at NREL, including projects with Chevron Corp. and the Colorado Center for Biofuels and Biorefining, and Darzins says in the extended future, both ways of producing algae will continue.

Back at It

NREL is currently experimenting with two algae production systems-in 270-liter ponds in a greenhouse, and small bioreactors that hold media to grow algae in artificial light with CO2. "When we start scaling both up to the commercial realm, though, that's where the debate lies," Darzins says. "It's an argument that has been heated for the past several years."

When generating large amounts of algae outside in closed photobioreactors, conventional wisdom is that the materials that go into making them are going to be cost-prohibitive unless the fuel produced is cheap, according to Darzins. "If you're making a value-added product that is worth a lot of money, then it might make sense to grow the algae in a closed photobioreactor," he says. "Right now, most people think the cost-effective way will be open raceway ponds, but there are some companies such as Solix that are growing their organisms in kind of a hybrid cultivation technology. Solazyme is growing algae not with sunlight, but within closed fermentation tanks with sugar. Under those conditions you can get very high cell densities and very high amounts of oil produced, but the main questions are, will that be cost effective and can you scale it up to be meaningful enough to displace the 40 billion-odd gallons of diesel we use here in the U.S.? Where are you going to get your cheap sugars to let your algae grow?"

Some believe once lignocellulosic ethanol technology is mature, the sugars extracted from corn stover and energy crops could be fed to bioreactors to reduce the cost of algae production. "That technology isn't quite there yet either," Darzins says. "There are a lot of different technologies that people are exploring but overall, the predominant method right now is open ponds."

Darzins believes if someone can develop truly novel bioreactors that are inexpensive to make and maintain while isolating organisms that are very productive, then it might make sense to grow algae that way, especially in more northern latitudes. On whether genetically modified organisms are productive or not, Darzins isn't sold. "We think Mother Nature has been engineering biology for millions and millions of years and there are some very interesting organisms that we just need to discover," he says. "Over the past four years, algal biofuels have captured the public and scientific communities attention and [its viability] really depends on whether we can produce it cost effectively and sustainably, from the aspects of land usage, water usage and nutrient usage; we just have to make sure all that can be done without competing with agriculture. We haven't heard the last of this debate, that's for sure."